White organic light emitting device

ABSTRACT

A white organic light emitting device includes: first and second electrodes formed to face each other on a substrate; a first stack configured with a hole injection layer, a first hole transportation layer, a first light emission layer and a first electron transportation layer which are stacked between the first and second electrodes; a second stack configured with a second hole transportation layer, a second light emission layer, a third light emission layer, a second electron transportation layer and an electron injection layer which are stacked between the first stack and the second electrode; and a charge generation layer interposed between the first and second stacks and configured to adjust a charge balance between the two stacks.

The present application claims priority under 35 U.S.C. §119(a) ofKorean Patent Application No. 10-2012-0148300 filed on Dec. 18, 2012 andKorean Patent Application No. 10-2013-0114553 filed on Sep. 26, 2013,which are hereby incorporated by references in their entireties.

BACKGROUND

1. Field of the Disclosure

The present application relates to an organic light emitting device, andmore particularly to a white organic light emitting device adapted toenhance efficiency and color reproduction range and reduce powerconsumption.

2. Description of the Related Art

Nowadays, a display field for visually representing an electricalinformation signal has been rapidly developed with the spread ofsubstantial information society. In accordance therewith, a variety offlat panel display devices with features, such as slimness, light weightand low power consumption have been developed. Also, the flat paneldisplay devices have been rapidly replacing the existing cathode raytubes (CRTs).

As examples of the flat panel display devices, liquid crystal display(LCD) devices, plasma display panel device (PDPs), field emissiondisplay (FED) devices, organic light emitting display (OLED) devices andso on can be introduced. Among the flat panel display devices, the OLEDdevice is being considered as the most competitive application. Thisresults from the fact that the OLED device without any separated lightsource can compact its applied appliance and display vivid colors.

It is necessary for the OLED device to form an organic light emissionlayer. The organic light emission layer is formed using a depositionmethod of the related art which employs a shadow mask.

However, a large-sized shadow mask can be sagged due to its load (or itsweight). Due to this, a fault can be generated the formed organic lightemission layer pattern. Also, the shadow mask cannot be used severaltimes. In view of this point, it is necessary to provide alternativeproposals.

As one of several methods for removing the shadow mask, a white OLEDdevice has been proposed. The white OLED device will now be described.

The white OLED device enables a plurality of layers between an anode anda cathode to be deposited without any mask at the formation of a lightemitting diode. In other words, an organic light emission layer and theother organic films, which include different materials, are sequentiallydeposited in a vacuum state. Such a white OLED device is used forvarious purposes, such as a thin light source, a backlight of the LCDdevice, a full-color display device with color filters or others.

The ordinary white OLED device uses a phosphorescent/fluorescent stackstructure. The phosphorescent/fluorescent stack structure includes afirst stack using a light emission layer of a blue fluorescent materialand a second stack which is formed on the first stack and uses anotherlight emission layer of a yellow-green phosphorescent material. Such awhite OLED device can realize white light by mixing blue light emittedfrom the blue fluorescent mater and yellow light (or yellow and greenlights) emitted from the yellow-green phosphorescent material.

However, it is necessary (or difficult) for the white OLED device of thephosphorescent/fluorescent stack structure to enhance luminousefficiency and color reproduction range and reduce power consumption.

BRIEF SUMMARY

Accordingly, embodiments of the present application are directed to awhite organic light emitting device that substantially obviates one ormore of problems due to the limitations and disadvantages of the relatedart.

The embodiments are to provide a white organic light emitting devicewhich includes first and second stacks or first through third stacks andenables a light emission layer to have a three-peaked luminous property.

Also, the embodiments are to provide a white organic light emittingdevice which is easy to inject holes by enabling a host of a lightemission layer of a second stack adjacent to a hole transportation layerto be formed the same material as the hole transportation layer.

Moreover, the embodiments are to provide a white organic light emittingdevice that is adapted to enhance panel efficiency and colorreproduction range and reduce power consumption by optimizing thicknessand doping density of a light emission layer of a second stack.

Furthermore, the embodiments are to provide a white organic lightemitting device with first through third stacks that is adapted toenhance a red brightness achievement ratio and color reproduction rangeby forming a light emission layer of the second stack in adouble-layered structure.

Additional features and advantages of the embodiments will be set forthin the description which follows, and in part will be apparent from thedescription, or may be learned by practice of the embodiments. Theadvantages of the embodiments will be realized and attained by thestructure particularly pointed out in the written description and claimshereof as well as the appended drawings.

According to an general aspect of the present embodiment for solvingproblems of the related art, a white organic light emitting deviceincludes: first and second electrodes formed to face each other on asubstrate; a first stack configured with a hole injection layer, a firsthole transportation layer, a first light emission layer and a firstelectron transportation layer which are sequentially stacked between thefirst and second electrodes; a second stack configured with a secondhole transportation layer, a second light emission layer, a third lightemission layer, a second electron transportation layer and an electroninjection layer which are sequentially stacked between the first stackand the second electrode; and a charge generation layer interposedbetween the first and second stacks and configured to adjust a chargebalance between the two stacks. The first light emission layer of thefirst stack is a blue fluorescent light emission layer with asingle-peaked luminous property, and the second and third light emissionlayers of the second stack have a two-peaked luminous property and comein contact with each other. One of the second and third light emissionlayers is one of green and yellow-green phosphorescent light emissionlayer, and the other one of the second and third light emission layersis a red phosphorescent light emission layer.

Other systems, methods, features and advantages will be, or will become,apparent to one with skill in the art upon examination of the followingfigures and detailed description. It is intended that all suchadditional systems, methods, features and advantages be included withinthis description, be within the scope of the present disclosure, and beprotected by the following claims. Nothing in this section should betaken as a limitation on those claims. Further aspects and advantagesare discussed below in conjunction with the embodiments. It is to beunderstood that both the foregoing general description and the followingdetailed description of the present disclosure are exemplary andexplanatory and are intended to provide further explanation of thedisclosure as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the embodiments and are incorporated herein andconstitute a part of this application, illustrate embodiment(s) of thepresent disclosure and together with the description serve to explainthe disclosure. In the drawings:

FIG. 1 is a cross-sectional view showing a white organic light emittingdevice according to a first embodiment of the present disclosure;

FIG. 2 is a table comparatively illustrating experimental resultants ofwhite organic light emitting devices according to the present disclosureand the related art;

FIG. 3 is a data sheet illustrating luminous wavelength and intensityproperties of white organic light emitting device according to thepresent disclosure and the related art;

FIG. 4 is a data sheet illustrating experimental resultants inaccordance with a volume ratio of hosts of a green light emission layerincluded in a second stack of the present disclosure;

FIG. 5 is a data sheet illustrating experimental resultants inaccordance with a volume ratio of hosts of a red light emission layerincluded in a second stack of the present disclosure;

FIG. 6 is a data sheet illustrating experimental resultants inaccordance with a doping ratio of a green light emission layer includedin a second stack of the present disclosure;

FIG. 7 is a cross-sectional view showing a white organic light emittingdevice according to a second embodiment of the present disclosure;

FIG. 8 is a data sheet illustrating luminous wavelength and intensityproperties of white organic light emitting devices according to thepresent disclosure and the related art;

and

FIG. 9 is a table comparatively illustrating experimental resultants ofwhite organic light emitting devices according to the present disclosureand the related art.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Reference will now be made in detail to the embodiments of the presentdisclosure, examples of which are illustrated in the accompanyingdrawings. These embodiments introduced hereinafter are provided asexamples in order to convey their spirits to the ordinary skilled personin the art. Therefore, these embodiments might be embodied in adifferent shape, so are not limited to these embodiments described here.In the drawings, the size, thickness and so on of a device can beexaggerated for convenience of explanation. Wherever possible, the samereference numbers will be used throughout this disclosure including thedrawings to refer to the same or like parts.

FIG. 1 is a cross-sectional view showing a white organic light emittingdevice according to a first embodiment of the present disclosure.

Referring to FIG. 1, the white organic light emitting device accordingto a first embodiment of the present disclosure includes first andsecond electrodes 110 and 130 formed on a substrate 100 in such manneras to face each other. A first stack 200, a charge generation layer 120and a second stack 300 are interposed between the first and secondelectrodes 110 and 130. The first stack 200, the charge generation layer120 and the second stack 300 can be formed by being sequentially stackedon the first electrode 110. The first and second stacks 200 and 300include different color light emission layers from each other. Differentcolor lights emitted from the light emission layers of the first andsecond stacks 200 and 300 are mixed, thereby realizing white light.

The substrate 100 can become an insulation substrate. A thin filmtransistor (not shown) can be formed on the insulation substrate. Theinsulation substrate can be formed from one of glass, a metal material,a plastic material, polyimide (PI) and so on. The thin film transistorcan include a gate electrode, a semiconductor layer, a source electrodeand a drain electrode.

The first electrode 110 can be used as an anode electrode. Also, thefirst electrode 110 can be formed from a transparent conductivematerial. For example, the first electrode 110 can be formed from oneselected from a material group which includes ITO (indium-tin-oxide),IZO (indium-zinc-oxide) and ZnO.

The second electrode 130 can be used as a cathode electrode. Also, thesecond electrode 130 can be formed from a metal material with a low workfunction. For example, the second electrode 130 can be formed from oneselected from a group which includes Mg, Ca, Al, Al-alloys, Ag,Ag-alloys, Au and Au-alloys.

The charge generation layer 120 interposed between the first and secondstacks 200 and 300 can apply electrons to the first stack 200 and holesto the second stack 300. As such, the charge generation layer 120 canadjust (or control) charge balance between the first and second stacks200 and 300. Such a charge generation layer 120 can be formed in asingle layer. For example, the charge generation layer 120 can become athin metal layer, which is formed from aluminum Al or others, or atransparent electrode formed from ITO (indium-tin-oxide) or others. Thesingle layered charge generation layer can enable the device to besimply configured and easily manufactured.

Alternatively, the charge generation layer 120 can be formed in amulti-layered structure, such as a junction structure of an organicmaterial layer which is formed by doping dopants. The charge generationlayer 120 with the multi-layered structure can easily transport theelectrons and the holes. As such, luminous efficiency of the device canbe enhanced and the life span of the device can be extended. Forexample, a surface layer of the charge generation layer 120, which comesin contact with the first stack 200, can be doped with an N-type dopant,in order to smoothly transfer electrons. Also, the other surface layerof the charge generation layer 120, which comes in contact with thesecond stack 300, can be doped with a P-type dopant, in order tosmoothly transfer holes.

The first stack 200 includes a hole injection layer (HIL) 202, a firsthole transportation layer (HTL) 203, a first light emission layer 204and a first electron transportation layer (ETL) 205 which aresequentially stacked between the first electrode 110 and the chargegeneration layer 120.

The hole injection layer (HIL) 202 can be formed from a material withsuperior hole injection capacity. In order to smoothly inject holes, thehole injection layer (HIL) 202 can be doped with a P-type dopant.

The first light emission layer 204 can become a blue fluorescent lightemission layer. Also, the first light emission layer 204 can become asingle light emission layer with a single luminous peak. Such a firstlight emission layer 204 can include a single host doped with a bluefluorescent dopant. Alternatively, the first light emission layer 204can be configured with two hosts doped with a blue fluorescent dopant.As an example of the blue fluorescent dopant of the first light emissionlayer 204, a dopant with a wavelength band of the luminous peak (or, aluminous peak wavelength band) of about 420 nm˜490 nm can be used.

The second stack 300 includes a second hole transportation layer (HTL)302, a second light emission layer 303, a third light emission layer304, a second electron transportation layer (ETL) 305 and an electroninjection layer (TIL) 306 which are sequentially stacked between thecharge generation layer 120 and the second electrode 130.

The electron injection layer (EIL) 306 can be formed from a materialwith superior electron injection capacity. In order to smoothly injectelectrons, the electron injection layer (HIL) 306 can be doped with anN-type dopant.

The second stack 300 allows the second and third light emission layers303 and 304 to be formed without having any one of a charge generationand a buffer layer therebetween. In other words, the second and thirdlight emission layers 303 and 304 can come in contact with each other.The stacked second and third light emission layers 303 and 304 can forma single light emission layer stack 301.

The light emission layer stack 301 can be formed by stacking a redphosphorescent light emission layer and a green phosphorescent lightemission layer. Also, the light emission layer stack 301 can be formedin a double-layered structure with two luminous peaks. Alternatively,the light emission layer stack 301 can be formed by stacking a redphosphorescent light emission layer and a yellow-green phosphorescentlight emission layer.

If the second light emission layer 303 is the red phosphorescent lightemission layer, the third light emission layer 304 can become one of thegreen phosphorescent light emission layer and the yellow-greenphosphorescent light emission layer. On the contrary, when the secondlight emission layer 303 is one of the green phosphorescent lightemission layer and the yellow-green phosphorescent light emission layer,the third light emission layer 304 can become the red phosphorescentlight emission layer. Preferably, the second light emission layer 303becomes the red phosphorescent light emission layer and the third lightemission layer 304 becomes the green phosphorescent light emissionlayer.

The green phosphorescent light emission layer or the yellow-greenphosphorescent light emission layer can be formed by doping a greenphosphorescent dopant or a yellow-green phosphorescent dopant which hasa luminous peak wavelength band of about 500 nm˜580 nm. The redphosphorescent light emission layer can be formed by doping a redphosphorescent dopant with a luminous peak wavelength band of 580 nm˜680nm. A doping ratio of the green or yellow-green phosphorescent dopant ofthe green or yellow-green phosphorescent light emission layer can becomehigher than that of the red phosphorescent dopant of the redphosphorescent light emission layer.

The green or yellow-green phosphorescent light emission layer can beformed thicker compared to the red phosphorescent light emission layer.Preferably, the green or yellow-green phosphorescent light emissionlayer becomes three times thicker than the red phosphorescent lightemission layer.

The second light emission layer 303 can be formed directly on the secondhole transportation layer 302. In other words, the second light emissionlayer 303 can come in contact with the second hole transportation layer302. Also, the second light emission layer 303 can include two hostsdoped with one of red, green and yellow-green phosphorescent dopants.The two hosts can include a hole type host and an electron type host.The hole type host can be formed to have a range of about 20˜80 volume %with respect to all the hosts. On the other hand, the electron type hostcan be formed to have a range of about 80˜20 volume % with respect toall the hosts.

The hole type host can be formed from the same material as the secondhole transportation layer 302. If the hole type host comes in directcontact with the second hole transportation layer 302 and is formed fromthe same material as the second hole transportation layer 302, holes canbe easily injected into the second light emission layer 303.

The third light emission layer 304 formed on the second light emissionlayer 303 can include two hosts doped with one of green, yellow-greenand red phosphorescent dopants. The two hosts can include a hole typehost and an electron type host. The hole type host can be formed to havea range of about 20˜80 volume % with respect to all the hosts. On theother hand, the electron type host can be formed to have a range ofabout 80˜20 volume % with respect to all the hosts.

Also, one host of the third light emission layer 304 can be formed bydoping one of green, yellow-green and red phosphorescent dopants. Also,the doped host can be formed to have a bipolar property.

In this manner, the first stack 200 adjacent to the anode electrodeforms a fluorescent unit and the second stack 300 adjacent to thecathode electrode forms a phosphorescent unit. As such, luminousefficiency of the white organic light emitting device can be enhanced.

On the other hand, in order to reduce power consumption of the whiteorganic light emitting device and enhance panel efficiency, it isnecessary to optimize a volume ratio of the hosts within each of thesecond and third light emission layers, and thickness and doping densityof each light emission layer.

FIG. 2 is a table comparatively illustrating experimental resultants ofwhite organic light emitting devices according to the present disclosureand the related art. FIG. 3 is a data sheet illustrating luminouswavelength and intensity properties of white organic light emittingdevice according to the present disclosure and the related art.

FIGS. 2 and 3 comparatively illustrate a two-peaked panel (or luminous)efficiency and a color reproduction range of the white organic lightemitting device according to the related art with a three-peaked panel(or luminous) efficiency and the color reproduction range of the whiteorganic light emitting device according to the present disclosure. Thewhite organic light emitting device of the related art used in theexperiment includes a first stack with a blue light emission layer and asecond stack with a yellow-green light emission layer. The white organiclight emitting device of the present disclosure includes a first stackwith a red light emission layer and a second stack with a green organiclight emission layer. In FIGS. 2 and 3, a red luminous peak B (i.e., athird peak B) of three-peaked greenish has a stronger intensity thanthat of three-peaked reddish. On the other hand, a green luminous peak A(i.e., a second peak A) of three-peaked reddish has a stronger intensitycompared to that of three-peaked greenish.

As seen from the experimental resultants I, II and IV, the white organiclight emitting device of the present disclosure has a higher colorreproduction range by about 10%, compared to that of the related art.Particularly, the three-peaked greenish represents that the whiteorganic light emitting device of the present disclosure enhances thecolor reproduction range without reducing the panel (or luminous)efficiency. Also, in comparison with the experimental resultants I andV, it is evident that the white organic light emitting device of thepresent disclosure not only enhances the panel (or luminous) efficiencybut also reduce power consumption, with having the same colorreproduction range as that of the related art white organic lightemitting device.

In other words, the white organic light emitting device of the presentdisclosure can either provide a higher color reproduction range or havehigher panel (or luminous) efficiency and lower power consumption,compared to that of the related art. Moreover, the enhancement ratios ofthe color reproduction range and the panel (or luminous) efficiency andthe reduction ratio of power consumption become larger, as the intensityof the green luminous peak A becomes stronger.

Although it is not shown in the drawings, the thicker the red lightemission layer or the green light emission layer is formed, the higherthe red and green luminous peaks A and B become. As seen from FIG. 2, itis evident that the panel (or luminous) efficiency and the colorreproduction range are more enhanced as the green luminous peak Abecomes stronger (or larger). As such, it is preferable for the greenorganic light emission layer to form thicker than the red organic lightemission layer. More preferably, the green organic light emission layeris formed thrice as thick as the red organic light emission layer.

FIG. 4 is a data sheet illustrating experimental resultants inaccordance with a volume ratio of hosts of a green light emission layerincluded in a second stack of the present disclosure.

The experimental resultant of FIG. 4 relates to the green organic lightemission layer of the second stack which includes the electron and holetype hosts doped with a green dopant. In detail, FIG. 4 illustratesgreen and red luminous peaks A and B of the second stack in accordancewith the ratio of the electron and hole type hosts of the green organiclight emission layer.

In the experiment, the condition of the red organic light emission layeris fixed. If the hole type host of the green organic light emissionlayer has a larger volume % compared to the electron type host, thegreen luminous peak A has the largest intensity but the red luminouspeak B has the smallest intensity. On the contrary, when the volume % ofthe hole type host is small than that of the electron type host, thegreen luminous peak A has the smallest intensity but the red luminouspeak B has the largest intensity. As shown in FIG. 2, the panel (orluminous) efficiency and the color reproduction range are more enhancedas the intensity of the green luminous peak A becomes larger. As such,in order to more enhance the panel efficiency and the color reproductionrange compared to those of the related art, the intensity of the greenluminous peak A must become larger (or stronger). To this end, thevolume ration of the hole and electron type hosts can be properly set.

FIG. 5 is a data sheet illustrating experimental resultants inaccordance with a volume ratio of hosts of a red light emission layerincluded in a second stack of the present disclosure.

The experimental resultant of FIG. 5 relates to the red organic lightemission layer of the second stack which includes the electron and holetype hosts doped with a red dopant. In detail, FIG. 5 illustrates greenand red luminous peaks A and B of the second stack in accordance withthe ratio of the electron and hole type hosts of the red organic lightemission layer.

The condition of the green organic light emission layer is fixed for theexperiment. If the hole type host of the red organic light emissionlayer has a larger volume % compared to the electron type host, thegreen luminous peak A has the largest intensity but the red luminouspeak B has the smallest intensity. On the contrary, when the volume % ofthe hole type host is small than that of the electron type host, thegreen luminous peak A has the smallest intensity but the red luminouspeak B has the largest intensity. As shown in FIG. 2, the panel (orluminous) efficiency and the color reproduction range are more enhancedas the intensity of the green luminous peak A becomes larger. As such,in order to more enhance the panel efficiency and the color reproductionrange compared to those of the related art, the intensity of the greenluminous peak A must become larger (or stronger). To this end, thevolume ration of the hole and electron type hosts can be properly set.

FIG. 6 is a data sheet illustrating experimental resultants inaccordance with a doping ratio of a green light emission layer includedin a second stack of the present disclosure.

The experimental resultant of FIG. 6 relates to the green organic lightemission layer of the second stack which includes hosts doped with agreen dopant. In detail, FIG. 6 illustrates green and red luminous peaksA and B of the second stack in accordance with the doping ratio of thegreen dopant.

As shown in FIG. 9, the intensity of the green luminous peak A becomeslarger as the doping ratio of the green dopant becomes higher.Similarly, although it is not shown in the drawing, the intensity of thered luminous peak B becomes larger as the doping ration of the reddopant for the red organic light emission layer of the second stackbecomes higher. As shown in FIG. 2, the panel (or luminous) efficiencyand the color reproduction range are more enhanced as the intensity ofthe green luminous peak A becomes larger. As such, it is preferable forthe doping ratio of the green dopant of the green organic light emissionlayer to set higher than that of the red dopant of the red organic lightemission layer.

FIG. 7 is a cross-sectional view showing a white organic light emittingdevice according to a second embodiment of the present disclosure.

Referring to FIG. 7, the white organic light emitting device accordingto a second embodiment of the present disclosure includes first andsecond electrodes 110 and 130 formed on a substrate 100 in such manneras to face each other. A first stack 400, a first charge generationlayer 220, a second stack 500, a second charge generation layer 320 anda third stack 600 are interposed between the first and second electrodes110 and 130. The first stack 400, the first charge generation layer 220,the second stack 500, the second charge generation layer 320 and thethird stack 600 can be formed by being sequentially stacked on the firstelectrode 110.

The first through third stacks 400, 500 and 600 can include the samecolor light emission layer. Alternatively, the second stack 500 caninclude a different color light emission layer from those of the firstand third stacks 400 and 600. In this case, different color lightsemitted from the light emission layers of the first through third stacks400, 500 and 600 are mixed, thereby realizing white light.

For example, the first and third stacks 400 and 600 can each include alight emission layer emitting blue light. Also, the second stack 500 caninclude either another light emission layer emitting red andyellow-green lights, or still another light emission layer emitting redand green lights. Moreover, the first and third stacks 400 and 600 canbecome fluorescent units, and the second stack 500 can become aphosphorescent unit.

The substrate 100 can become an insulation substrate. A thin filmtransistor (not shown) can be formed on the insulation substrate. Theinsulation substrate can be formed from one of glass, a metal material,a plastic material, polyimide (PI) and so on. The thin film transistorcan include a gate electrode, a semiconductor layer, a source electrodeand a drain electrode.

The first electrode 110 can be used as an anode electrode. Also, thefirst electrode 110 can be formed from a transparent conductivematerial. For example, the first electrode 110 can be formed from oneselected from a material group which includes ITO (indium-tin-oxide),IZO (indium-zinc-oxide) and ZnO.

The second electrode 130 can be used as a cathode electrode. Also, thesecond electrode 130 can be formed from a metal material with a low workfunction. For example, the second electrode 130 can be formed from oneselected from a group which includes Mg, Ca, Al, Al-alloys, Ag,Ag-alloys, Au and Au-alloys.

The first charge generation layer 220 interposed between the first andsecond stacks 400 and 500 can apply electrons to the first stack 400 andholes to the second stack 500. As such, the first charge generationlayer 220 can adjust (or control) charge balance between the first andsecond stacks 400 and 500. Similarly, the second charge generation layer320 interposed between the second and third stacks 500 and 600 can applyelectrons to the second stack 500 and holes to the third stack 600. Assuch, the second charge generation layer 320 can adjust (or control)charge balance between the second and third stacks 500 and 600.

Also, the first and second charge generation layers 220 and 320 can beformed in a single layer. For example, each of the first and secondcharge generation layers 220 and 320 can become either a thin metallayer, which is formed from aluminum Al or others, or a transparentelectrode formed from ITO (indium-tin-oxide) or others. The first andsecond charge generation layers 220 and 320 with the single layerstructure can enable the device to be simply configured and easilymanufactured.

Alternatively, each of the first and second charge generation layers 220and 320 can be formed in a multi-layered structure, such as a junctionstructure of an organic material layer which is formed by dopingdopants. The first and second charge generation layers 220 and 320 withthe multi-layered structure can easily transport the electrons and theholes. As such, luminous efficiency of the device can be enhanced andthe life span of the device can be extended. For example, a surfacelayer of the first charge generation layer 220 contacting the firststack 400 and a surface layer of the second charge generation layer 320contacting the second stack 500 can be doped with an N-type dopant, inorder to smoothly transfer electrons. Also, the other surface layer ofthe first charge generation layer 220 contacting the second stack 500and the other surface layer of the second charge generation layer 320contacting the third stack 600 can be doped with a P-type dopant, inorder to smoothly transfer holes.

The first stack 400 includes a hole injection layer (HIL) 402, a firsthole transportation layer (HTL) 403, a first light emission layer 404and a first electron transportation layer (ETL) 405 which aresequentially stacked between the first electrode 110 and the chargegeneration layer 420. The hole injection layer (HIL) 402 can be formedfrom a material with superior hole injection capacity. In order tosmoothly inject holes, the hole injection layer (HIL) 402 can be dopedwith a P-type dopant.

The first light emission layer 404 can become a blue fluorescent lightemission layer. Also, the first light emission layer 404 can become asingle light emission layer with a single luminous peak. Such a firstlight emission layer 404 can include a single host doped with a bluefluorescent dopant. Alternatively, the first light emission layer 404can include two hosts which are doped with a blue fluorescent dopant. Asan example of the blue fluorescent dopant of the first light emissionlayer 404, a dopant with a wavelength band of the luminous peak (or, aluminous peak wavelength band) of about 420 nm˜490 nm can be used.

The second stack 500 includes a second hole transportation layer (HTL)502, a second light emission layer 503, a third light emission layer 504and a second electron transportation layer (ETL) 505 which aresequentially stacked between the first and second charge generationlayers 220 and 320. The second stack 500 allows the second and thirdlight emission layers 503 and 504 to be formed without having any one ofa charge generation and a buffer layer therebetween. In other words, thesecond and third light emission layers 503 and 504 can come in contactwith each other. The stacked second and third light emission layers 503and 504 can form a single light emission layer stack 501.

The light emission layer stack 501 can be formed by stacking a redphosphorescent light emission layer and a yellow-green phosphorescentlight emission layer. Also, the light emission layer stack 501 can beformed in a double-layered structure with two luminous peaks.Alternatively, the light emission layer stack 501 can be formed bystacking a red phosphorescent light emission layer and a greenphosphorescent light emission layer.

If the second light emission layer 503 is the red phosphorescent lightemission layer, the third light emission layer 504 can become one of thegreen phosphorescent light emission layer and the yellow-greenphosphorescent light emission layer. On the contrary, when the secondlight emission layer 503 is one of the green phosphorescent lightemission layer and the yellow-green phosphorescent light emission layer,the third light emission layer 504 can become the red phosphorescentlight emission layer. Preferably, the second light emission layer 503becomes the red phosphorescent light emission layer and the third lightemission layer 304 becomes the yellow-green phosphorescent lightemission layer.

The green phosphorescent light emission layer or the yellow-greenphosphorescent light emission layer can be formed by doping a greenphosphorescent dopant or a yellow-green phosphorescent dopant which hasa luminous peak wavelength band of about 500 nm˜580 nm. The redphosphorescent light emission layer can be formed by doping a redphosphorescent dopant with a luminous peak wavelength band of 580 nm˜680nm. A doping ratio of the green or yellow-green phosphorescent dopant ofthe green or yellow-green phosphorescent light emission layer can becomehigher than that of the red phosphorescent dopant of the redphosphorescent light emission layer.

The green or yellow-green phosphorescent light emission layer can beformed thicker compared to the red phosphorescent light emission layer.Preferably, the green or yellow-green phosphorescent light emissionlayer becomes three times thicker than the red phosphorescent lightemission layer.

The second light emission layer 503 can be formed directly on the secondhole transportation layer 302. In other words, the second light emissionlayer 503 can come in contact with the second hole transportation layer502. Also, the second light emission layer 503 can include two hostsdoped with one of red, green and yellow-green phosphorescent dopants.The two hosts can include a hole type host and an electron type host.

The hole type host can be formed to have a range of about 20˜80 volume %with respect to all the hosts. On the other hand, the electron type hostcan be formed to have a range of about 80˜20 volume % with respect toall the hosts. The hole type host can be formed from the same materialas the second hole transportation layer 502, in order to easily injectholes.

The third light emission layer 504 formed on the second light emissionlayer 503 can include two hosts doped with one of yellow-green, greenand red phosphorescent dopants. The two hosts can include a hole typehost and an electron type host. The hole type host can be formed to havea range of about 20˜80 volume % with respect to all the hosts. On theother hand, the electron type host can be formed to have a range ofabout 80˜20 volume % with respect to all the hosts.

Also, one host of the third light emission layer 504 can be formed bydoping one of green, yellow-green and red phosphorescent dopants. Also,the doped host can be formed to have a bipolar property.

The third stack 600 includes a third hole transportation layer (HTL)601, a fourth light emission layer 602, a third electron transportationlayer (ETL) 603 and an electron injection layer (TIL) 604 which aresequentially stacked between the second charge generation layer 320 andthe second electrode 130. The electron injection layer (EIL) 604 can beformed from a material with superior electron injection capacity. Inorder to smoothly inject electrons, the electron injection layer (HIL)604 can be doped with an N-type dopant.

The fourth light emission layer 602 can become a blue fluorescent lightemission layer. Also, the fourth light emission layer 602 can become asingle light emission layer with a single luminous peak. Such a fourthlight emission layer 602 can include a single host doped with a bluefluorescent dopant. Alternatively, the fourth light emission layer 602can be configured with two hosts doped with a blue fluorescent dopant.As an example of the blue fluorescent dopant of the fourth lightemission layer 602, a dopant with a luminous peak wavelength band ofabout 420 nm˜490 nm can be used.

FIG. 8 is a data sheet illustrating luminous wavelength and intensityproperties of white organic light emitting device according to thepresent disclosure and the related art.

FIG. 8 comparatively illustrates luminous wavelength and intensityproperties of the white organic light emitting devices according to therelated art and the present disclosure. The white organic light emittingdevice of the related art used in the experiment includes first andthird stacks with blue light emission layers and a second stack with ayellow-green light emission layer. The white organic light emittingdevice of the present disclosure includes first and third stacks withblue light emission layers and a second stack with a red organic lightemission layer and a yellow-green organic light emission layer.

Referring to a red domain represented by a dotted circle in FIG. 8, thewhite organic light emitting device of the related art generates onlytwo luminous peaks, because any luminous peak is not generated in thered domain. Meanwhile, the white organic light emitting device of thepresent disclosure provides a three-peaked luminous property. In otherwords, the white organic light emitting device of the present disclosureenables the intensity in the red domain to become larger, compared tothat of the related. A brightness achievement ratio, the panelefficiency and the color reproduction range will now be described indetail.

FIG. 9 is a table comparatively illustrating experimental resultants ofwhite organic light emitting devices according to the present disclosureand the related art.

Referring to FIG. 9, if a brightness target value is set to be 100%, thewhite organic light emitting device of the related art can obtain a redbrightness achievement ratio of 88%. In other words, the white organiclight emitting device of the related art is short of red brightness.This results from the fact that a separated red light emission layerdoes not exist and any luminous peak is not generated in the red domain.

Meanwhile, the white organic light emitting device of the presentdisclosure allows the second stack to include the light emission layerstack which is formed by stacking a red organic light emission layer anda yellow-green organic light emission layer. As such, the white organiclight emitting device of the present disclosure can obtain a strongerluminous intensity, compared to that of the related art.

In comparison with brightness achievement ratios in accordance with therelated art and the present disclosure, the white organic light emittingdevice of the present disclosure can obtain the red brightnessachievement ratio of 101% which is higher by 13%. In other words, thewhite organic light emitting device of the present disclosure canprovide a larger red brightness achievement ratio than the brightnesstarget value of 100%. Also, it is evident that green and blue brightnessachievement ratios of the white organic light emitting device accordingto the present disclosure are in excess of the brightness target valueof 100%.

Moreover, it is evident that the panel efficiency of the white organiclight emitting device of the present disclosure is equal to or largerthan that of the related art. Furthermore, the color reproduction rangeincreases as the luminous intensity in the red wavelength domain becomesstronger.

In this manner, the white organic light emitting device according to thepresent disclosure configured with first and second stacks or firstthrough third stacks can provide a three-peaked luminous property. Also,because the light emission layer of the second stack adjacent to thehole transportation layer is formed from the same material as the holetransportation layer, it can be easy to inject holes. Moreover, thethickness and the doping density of the light emission layer within thesecond stack are optimized. In accordance therewith, the panelefficiency and the color reproduction range can be enhanced, and powerconsumption can be reduced. Furthermore, the white organic lightemitting device with the first through third stacks enables the organiclight emission layer of the second stack to be formed in adouble-layered structure. As such, the red brightness achievement ratioand the color reproduction range can increase.

Although the present disclosure has been limitedly explained regardingonly the embodiments described above, it should be understood by theordinary skilled person in the art that the present disclosure is notlimited to these embodiments, but rather that various changes ormodifications thereof are possible without departing from the spirit ofthe present disclosure. Accordingly, the scope of the present disclosureshall be determined only by the appended claims and their equivalentswithout being limited to the detailed description.

What is claimed is:
 1. A white organic light emitting device comprising:first and second electrodes formed to face each other on a substrate; afirst stack configured with a hole injection layer, a first holetransportation layer, a first light emission layer and a first electrontransportation layer which are sequentially stacked between the firstand second electrodes; a second stack configured with a second holetransportation layer, a second light emission layer, a third lightemission layer, a second electron transportation layer and an electroninjection layer which are sequentially stacked between the first stackand the second electrode; and a charge generation layer interposedbetween the first and second stacks and configured to adjust a chargebalance between the first and second stacks, wherein the first lightemission layer of the first stack is a blue fluorescent light emissionlayer with a single-peaked luminous property, the second and third lightemission layers of the second stack have a two-peaked luminous propertyand come in contact with each other, one of the second and third lightemission layers is one of green and yellow-green phosphorescent lightemission layer, and the other one of the second and third light emissionlayers is a red phosphorescent light emission layer.
 2. The whiteorganic light emitting device of claim 1, wherein the second lightemission layer comes in direct contact with the second holetransportation layer and includes hole and electron type hosts dopedwith a dopant.
 3. The white organic light emitting device of claim 2,wherein the hole type host of the second light emission layer is formedfrom the same material as the second hole transportation layer.
 4. Thewhite organic light emitting device of claim 2, wherein the hole typehost of the second light emission layer is formed in a range of 20˜80volume % with respect to all the hosts.
 5. The white organic lightemitting device of claim 1, wherein the third light emission layerincludes hole and electron hosts doped with a dopant.
 6. The whiteorganic light emitting device of claim 5, wherein the hole type host ofthe third light emission layer is formed in a range of 20˜80 volume %with respect to all the hosts.
 7. The white organic light emittingdevice of claim 1, wherein the third light emission layer includes abipolarity host doped with a dopant.
 8. The white organic light emittingdevice of claim 1, wherein the green phosphorescent light emission layeris thicker than the red phosphorescent light emission layer.
 9. Thewhite organic light emitting device of claim 8, wherein the greenphosphorescent light emission layer is thrice as thick as the redphosphorescent light emission layer.
 10. The white organic lightemitting device of claim 1, wherein the green phosphorescent lightemission layer is higher than the red phosphorescent light emissionlayer in a doping ratio of the dopant.
 11. A white organic lightemitting device comprising: first and second electrodes formed to faceeach other on a substrate; a first stack interposed between the firstand second electrode and configured with a hole injection layer, a firsthole transportation layer, a first light emission layer and a firstelectron transportation layer which are sequentially stacked on thefirst electrode; a second stack interposed between the first stack andthe second electrode and configured with a second hole transportationlayer, a second light emission layer, a third light emission layer and asecond electron transportation layer which are sequentially stacked onthe first stack; a third stack interposed between the second stack andthe second electrode and configured with a third hole transportationlayer, a fourth light emission layer, a third electron transportationlayer and an electron injection layer which are sequentially stacked onthe second stack; and charge generation layers interposed between thefirst and second stacks and between the second and third stacks andconfigured to adjust a charge balance between the stacks, wherein eachthe first and fourth light emission layers is a blue fluorescent lightemission layer with a single-peaked luminous property, the second andthird light emission layers of the second stack have a two-peakedluminous property and come in contact with each other, one of the secondand third light emission layers is one of green and yellow-greenphosphorescent light emission layer, and the other one of the second andthird light emission layers is a red phosphorescent light emissionlayer.
 12. The white organic light emitting device of claim 11, whereinthe second light emission layer comes in direct contact with the secondhole transportation layer and includes hole and electron type hostsdoped with a dopant.
 13. The white organic light emitting device ofclaim 12, wherein the hole type host of the second light emission layeris formed from the same material as the second hole transportationlayer.
 14. The white organic light emitting device of claim 12, whereinthe hole type host of the second light emission layer is formed in arange of 20˜80 volume % with respect to all the hosts.
 15. The whiteorganic light emitting device of claim 11, wherein the third lightemission layer includes hole and electron type hosts doped with adopant.
 16. The white organic light emitting device of claim 15, whereinthe hole type host of the third light emission layer is formed in arange of 20˜80 volume % with respect to all the hosts.
 17. The whiteorganic light emitting device of claim 11, wherein the third lightemission layer includes a bipolarity host doped with a dopant.
 18. Thewhite organic light emitting device of claim 11, wherein the greenphosphorescent light emission layer is thicker than the redphosphorescent light emission layer.
 19. The white organic lightemitting device of claim 18, wherein the green phosphorescent lightemission layer is thrice as thick as the red phosphorescent lightemission layer.
 20. The white organic light emitting device of claim 11,wherein the green phosphorescent light emission layer is higher than thered phosphorescent light emission layer in a doping ratio of the dopant.